Structural inspections represent a large portion of overall maintenance costs on aircraft and other vehicles and structures. Non-destructive inspection (NDI) of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is often preferred over visual or destructive inspection methods to avoid the schedule, labor, and costs associated with removal of parts or other disassembly for inspection (with the associated potential for damaging the structure). In the field, access to interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor. Frequently, inspections are necessary or mandated to be performed in hazardous or difficult-to-access areas, such as in fuel cells, electronics bays, and pressure bulkhead cavities, which may require fuel cell venting and removal of panels, ducts, insulation, and other surrounding structures. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required, particularly where gaining access to an inspection area is limited. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for damage or defects (flaws) in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into service to validate the integrity and fitness of the structure.
Related to the need for performing structural inspections is the ability to determine whether maintenance is required. For example, to decrease the costs of airplane maintenance, the concept of Vehicle Health Management (VHM) can be used to more accurately determine when maintenance is required, in essence by monitoring the health of the vehicle. Central to the concept of Vehicle Health Management for an airplane is a network of sensors installed throughout the airplane. These sensors may be monitored continuously or queried periodically during maintenance checks, when the tools and/or facilities for repairing any problems are immediately available. Such a sensor network may also be used for Condition-Based Maintenance (CBM) in which maintenance checks and maintenance of parts and systems of a vehicle may be initiated by sensor data. For example, maintenance intervals of airplanes are typically conservatively set for routine maintenance, but Condition-Based Maintenance could reduce the need for certain routine maintenance which can be monitored to determine when the maintenance is required, thereby resulting in less frequent maintenance and reduced maintenance overall.
A feature of airplanes and other vehicles and structures that is critical to structural integrity is the bolted joint, including the bolted composite joint. Of concern for bolted composite joints is the potential for defects such as delamination and fatigue cracking around the bolt-holes. Currently, one approach for inspecting bolted joints involves an operator gaining access to an inspection area around the bolted joint and inspecting the area immediately surrounding the bolt-hole with a shear wave ultrasonic beam or eddy current. The operator typically scans the inspection area while monitoring a display screen for any signals which may be interpreted as a defect in the structure. Such inspection often requires preparing the inspection area surface, such as scraping away sealant fillets to provide a clean surface for a transducer or probe. Gaining access to the inspection area often involves removing access panels, hydraulic lines, cables, hoses, brackets, and other interfering structures. In addition to the practical impediments to such inspection, the operator must know how to place and orient a transducer or probe to ensure that defects are examined from the optimum angle. Further, the operator must be able to interpret and evaluate the inspection data on the display in real-time and determine if any potential defects are significant or non-significant. Following such inspections, sealants and coatings must be restored and interfering structures replaced.
Manual inspection of structures typically is very labor intensive, time consuming, and expensive. Manual inspection is subject to human error in performance and variations of interpretation of results. Noise in inspection signals can be interpreted as defects (false positives), and defects can be missed or overlooked as non-significant (false negatives). Further, shear wave ultrasonic beam and eddy current inspection are limited in that only cracks of particular orientations may be detectable. Many structures may also incorporate numerous bolted joints which require inspection in areas which cannot be accessed or are exceptionally difficult to access.
Several approaches have been attempted to inspect bolted joints with sensors. One approach is a smart washer proposed by Innovative Dynamics, Inc, of Ithaca, N.Y. These smart washers incorporate eddy current sensors. However, the sensors cannot be “nulled” or balanced between widely spaced interrogation intervals, so it is not possible to discern crack signals from signals caused by temperature variations, instrument drift, and other noise factors. Furthermore, these smart washers use eddy currents and can only be used on electrically conductive structures.
Another approach is using eddy current rosettes produced by Jentek Sensors of Waltham, Mass. These eddy current rosettes are bonded onto the area surrounding a rivet or bolt. The rosettes contain eddy current sensor loops for detection of surface-breaking cracks. The rosettes can be calibrated in air and provide an absolute measurement, unlike the Innovative Dynamics smart washers which only provide relative measurements that depend upon a stable null point over time. However, the Jentek Sensors rosettes depend on a strain gage adhesive to cement the sensor in place, and these adhesives are subject to failure over time. Furthermore, the Jentek Sensors rosettes are expensive, can be difficult to use and understand, and require new computer models for different applications.
Yet another approach is comparative vacuum monitoring (CVM) sensors produced by Structural Monitoring Systems Ltd. of Perth, Australia. Comparative vacuum monitoring sensors measure the pressure differential between small recesses containing a low vacuum alternating with small recesses at atmosphere, where the alternating series of low vacuum and atmosphere recesses are located in a simple manifold. If no surface-breaking crack is present, the low vacuum will remain at a stable level. If a crack develops, air will flow through the crack from the atmosphere recesses to the vacuum recesses. Comparative vacuum monitoring sensors only have application to surface-breaking fatigue cracks and are unable to detect delaminations below the surface, or fatigue cracks originating at the far surface of a layer.
Although used for a different purpose, a related technology is the permanent mounted transducer (PMT) system by PFW Technologies GmbH of Speyer, Germany. The PFW Technologies permanent mounted transducer system uses an ultrasonic transducer with a bolt to measure elongations of the bolt caused by the clamp load on the bolt, thereby providing a way of measuring the clamp load on the bolt during assembly and providing a way of controlling the torque load exerted by a tightening tool. The PFW Technologies permanent mounted transducer system, however, does not have the capability of detecting defects in the vicinity of the bolt-hole, but only monitors changes in the bolt stress state.
Accordingly, improved apparatus, systems, and methods for inspecting bolted joints of structures are desired.